TTY devices—also known as TDDs or Text Telephones—are text terminals commonly employed by people who are deaf or hard-of-hearing in order to communicate over telephone systems. A problem within the field is that, often depending on the country of origin, there are presently at least nine different incompatible TTY transmission formats. These include:                (1) 45.45 baud Baudot tones (common in the US)        (2) The Bell 103 modem standard (common in the US)        (3) The Bell 202 modem standard (common in the US)        (4) Turbocode (a proprietary TTY protocol developed by Ultratec Corporation)        (5) European Deaf Telephone, or EDT (common in, e.g., Germany, Austria, Italy, and Spain)        (6) Minitel (common, e.g., in France)        (7) ITU V.21 modem (common, e.g., in Sweden, Finland, Norway, and the UK)        (8) 50 baud Baudot tones (common, e.g., in Ireland and Australia)        (9) Dual tone multi-frequency signals, or DTMF (common, e.g., in Holland and Denmark)        
In all cases, the audio-encoded signals generated by these TTY devices are transported reliably by the traditional analog Circuit Switched Telephone Networks (CSTN's) such as the Switched Telephone Network (CSTN), or networks of circuit-switched Private Branch Exchanges (PBX's), or combination thereof. Unfortunately, the same does not hold true for Voice over Internet Protocol (VoIP) telephony systems. When TTY signals are transported on VoIP networks, using the same mechanisms that are used for voice transmissions, common sources of VoIP audio distortion that tend to be tolerated by the human ear—notably audio compression and packet loss—can have a devastating impact on TTY transmission accuracy. For this reason, it is generally accepted that the gateways through which VoIP networks interface with the CSTN must be able to translate between analog TTY signals on the CSTN and non-audio data packets suitable for reliable transmission within VoIP networks.
Although many transmission formats have been proposed for TTY-on-IP, international standards organizations appear to be leaning toward adopting ITU Recommendation T.140 (“Text Conversion Protocol for Multi-Media Application”), with RFC-4103 as the payload type. Within this proposed scenario, VoIP endpoints that are capable of TTY-style interaction would be required to support the T.140/RFC-4103 standard. It is important to note that these VoIP endpoints would not be expected to recognize or transmit traditional CSTN TTY signals. When the user of a T.140/RFC-4103 VoIP device is interacting with the user of a traditional TTY device on the CSTN, the expectation is that the VoIP/CSTN gateway will translate between the two devices.
Looking now at telecommunications laws and regulations that apply to TTY usage, such as the Americans with Disabilities Act of 1990, Section 255 of the Telecommunications Act of 1996, and Section 508 of the Rehabilitation Act Amendments of 1998, it is clearly desirable (if not a requirement) that TTY users be permitted to interact with each other directly, without regard for whether they are on the CSTN or are using VoIP. Although the problem is solvable by enabling all ports on VoIP/CSTN gateways to translate between T.140/RFC-4103 packets and traditional analog TTY signals, such an approach is economically impractical. Keeping in mind that TTY conversations constitute a relatively small percentage of all telephone calls, it is hard to justify the added complexity and cost of requiring all VoIP/CSTN gateway ports to “listen for” and be able to translate all nine of the above CSTN TTY protocols.